import numpy as np
import matplotlib.pyplot as plt


# 绘制向量图

n = 8
X, Y = np.mgrid[0:n, 0:n]
T = np.arctan2(Y - n/2.0, X - n/2.0)
R = 10 + np.sqrt((Y - n/2.0)**2 + (X - n/2.0)**2)
U, V = R* np.cos(T), R* np.sin(T)

plt.axes([0.025, 0.025, 0.95, 0.95])
plt.quiver(X, Y, U, V, R, alpha=.5)
plt.quiver(X, Y, U, V, edgecolor='k', facecolor='None', linewidth=.5)

plt.xlim(-1, n), plt.xticks([])
plt.ylim(-1, n), plt.yticks([])

# savefig('../figures/quiver_ex.png',dpi=48)
plt.show()


# https://matplotlib.org/stable/api/_as_gen/matplotlib.pyplot.quiver.html
